Techniques have been known for changing the position and altitude of the seat surface and the like of a driver's seat in a vehicle according to the body shape and likings of the driver. They aim at improving the driver's feel of seating by utilizing such a driver's seat. On the other hand, as performances of vehicles have been improving, high-speed running and operating characteristics have been made better than those conventionally attained. As a result, the acceleration, deceleration, rolling force, and the like acting on passengers have been becoming greater than conventional ones. As the speed and posture of the vehicle change, the inertial force acting on the passengers may alter the state of seating of the passengers, so as to cause a sense of discomfort in the feel of seating, thereby making the passengers feel uncomfortable.
Known as a technique for restraining the seating state from changing according to such a vehicle behavior is one disclosed in Japanese Patent Application Laid-Open No. 5-85235. This technique detects the amount of displacement of the posture of a vehicle and displaces the posture of a seat in response thereto, so as to compensate for the inertial force occurring in the passengers, thereby reducing or eliminating the sense of discomfort in the passengers.